In the drilling of wells from a vessel at an offshore location it is necessary that a riser or elongated conductor extend from the vessel to the ocean floor, being normally connected to the well head structure. The function of the riser is to enclose the drill string and permit circulation of the drilling mud and drilling fluids during a drilling operation. Normally the riser comprises a series of pipe-like elements which are sealably joined into an elongated single conduit.
It can be appreciated that in the instance of relatively deep waters, the riser can be subjected to extreme stresses. This normally results from the action of water currents and the movement of the drilling vessel at the water's surface.
For example, such as during a Tsunami or hurricane the riser can be subjected to water currents in more than one direction. This action will induce a number of curves and stresses into the riser structure. The problem however can be minimized or even obviated by the use of suitable tensioning apparatus on the drilling vessel. Such apparatus functions to stress the riser to a predetermined degree so that the amount of physical deformation is minimized.
In relatively deep waters the necessary use of risers has imposed a number of problems which increase in intensity with water depth. However, where the waters are infested with hurricanes, storms, natural disasters, and the like, it can be appreciated that these stresses are greatly amplified on the riser.
For example, in waters subject to typhoons, it is necessary to quickly move the drilling or production vessel out of the area to be affected by the storm. The notice of such a storm is usually about 24 hours, leaving very little time to disconnect the drilling or production vessel and move it to a safe location. A drilling or production unit that can be quickly and easily removed from the riser would be highly desirable and cost effective.
Toward minimizing the time consumed to detach the vessel, and to minimize the expense of such a deep water drilling operation, the present invention provides a system wherein a drilling vessel is connected at the ocean floor by way of a disconnectable riser. The latter is provided with at least one remotely actuated connecting joint.
Functionally, the connecting joint is positioned in the riser structure approximately fifty to five-hundred feet (50'-500') below the water's surface in the instance of water depths in excess of about 1,000 feet. By uncoupling the riser at the joint, the upper segment can be displaced with the drill vessel while the lower segment remains substantially in place buoyed with a gas filled canister. The upper end of the detached segment is at a sufficient depth below the water's surface to be safe from damage as the storm passes.
It is therefore an object of the invention to provide an offshore well drilling and or production system capable of being rapidly disconnected from a drilling or production vessel such that the vessel can be removed quickly from the system. A further object is to provide such a system which is capable of permitting the riser member to be rapidly disconnected under emergency conditions at a point below the water's surface so that at least part of the riser will be displaced and the remainder held uprightly in place. A still further object is to provide a drill riser of the type contemplated which is adapted to be disconnected at such time as the drilling vessel is removed, and is further adapted to be readily reconnected at such time as the drilling vessel returns to recommence a drilling or production operation, either manually or by remote means.